Garelli Cylinder timing/porting?

I've read Jennings' Two Stroke tuning book and found the port-time-area timing chapter quite interesting.

I've measure port opening angles, but yet to remove the barrel to measure the port areas.

Has anybody mapped a '30mph' horizontal Garelli cylinder? If so, how do the port-time-area timings compare with Jennings' suggestions?

I'm tempted to widen & smooth the ports if required, although will probably not change durations.

Has anybody done any radical exhaust port raising? What were the results?

Re: Garelli Cylinder timing/porting?

Its all a trade off, too much roof raising is gonna make your low RPM performance suck, While it will allow for more revs on the high side. I've studied some cylinders from 2 stroke motorcycles, with manual clutches, the exhaust port timing is right at 170-180 degrees duration. most moped cylinders I've look at are between 120-130 degrees open duration. I bring up the manual clutch issue only because with a manual you can rev the bike into its power band before engaging the transmission. on a moped you have to try to tune your clutch to engage later, seems easy enough to do on Puch clutches, i don't know how the Garelli setup works or if can be altered

Re: Garelli Cylinder timing/porting?

It's been awhile since I peered into a NOI cylinder but I remember being taken a back by the unusual port timing. Can't remember what seemed askew. Might be why stock they are so slow

Re: Garelli Cylinder timing/porting?

I'm not sure how unusual they are, but I measured the following:

Exhaust duration 130 degrees

dual Transfer(? into piston) ports duration 108 degrees

The Garelli rubber clutch bites at very low rpm, which doesn't help take-off acceleration.

Re: Garelli Cylinder timing/porting?

I'd leave the duration alone and if anything widen the exhaust as much as possible (60% of bore diam??) this will allow your exhaust to exit faster/more/better at higher RPMs while not robbing power from the working part of the stroke at low RPMs

Take the jug off and do a port map like jennings describes and compare the time/area values to the chart and you can see the highest RPM range that it breath right

Re: Garelli Cylinder timing/porting?

fish_SHARK /

a man once told me to raise the exhaust by 1mm

Re: Garelli Cylinder timing/porting?

"I’d leave the duration alone and if anything widen the exhaust as much as possible (60% of bore diam??)"

That's the sort of thing I had in mind.

Has anybody experienced porting Garellis then?

Re: Garelli Cylinder timing/porting?

no experience but i plan to port soon. so if you do it before me .. take some photos and measurements.. and visa versa

Re: Garelli Cylinder timing/porting?

Anybody got any more thoughts on this before I bite the bullet?

Re: Garelli Cylinder timing/porting?

Read about port mapping.. take one of them and study it.

Re: Garelli Cylinder timing/porting?

Well, the cylinder tweaking is getting there.

I took measurements and compared with Jennings. The ports are on the small side, but with the grabby Garelli clutch I don't want to lose all of the bottom end so I've been conservative.

The transfers have been cleaned up with sharp edges removed.

The exhaust port is now 2mm wider, the top chamfered slightly, the lumps in the casting ground down and the whole port blended in and polished up nicely.

I've cleaned up the intake port and port-matched with the intake -quite a bit of difference- and discovered that the crankcase and cylinder are already well-matched.

I've also de-coked the head and will clean up the piston before re-assembly.

Dremels are a wonderful thing.

If there's a noticeable improvement in performance I may take the porting a bit further and overhaul it completely.

Re: Garelli Cylinder timing/porting?

some things i've learned about garelli noi:

they have really aggressive porting stock. i think there are a couple different cylinders, but the one i ported came out to a lil' over 135 deg exhaust. The transfers are huge, exhaust is huge, they mostly need intake port enlargening, and a bigger manifold. should be pretty easy to make a mani, drill the carb, and hog out that intake. at least 1 mm drop and a mm or 2 on each side wider. there are no rings to snag, so have fun. If you do hack on an exhaust, i think for stock the tomos 4p would be killer, for kitted, tomos tecno estoril.

gears are hard to find, and the clutch is a pita, but you can worry about that later

Re: Garelli Cylinder timing/porting?

Good info. My exhaust port started off at 130 degrees, which I'd consider a short duration compared with a motorbike.

The intake port does seem quite small, although I'll make do with tidying it up and matching with the manifold at the manifold.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account